Digital Subscriber Lines (DSL) are a common technology for providing digital communication over copper twisted pair subscriber lines. The term DSL (or sometimes xDSL) is an umbrella term for several similar technologies, such as ADSL, ADSL2, ADSL2+, VDSL, VDSL2, G.fast, etc.
In DSL, the frequency range is divided up into a number of carriers (sometimes referred to as bins) for carrying data bits. Typically, each carrier has the same amount of bandwidth but transmits on its own unique frequency. This transmit frequency is known as the “tone”, and also acts as an identifier for its particular carrier. DSL technology has generally evolved to use wider frequency ranges to accommodate a greater level of tones and bit loading, thus giving users a greater data rate. Most standards for the various DSL technologies do not include the concept of channelization, so the whole frequency range is notionally a single channel with global configuration settings applied to it. This theoretically could compromise the operation of certain services which have distinct transmission requirements on the line (for example, Voice-over-IP (VoIP) services generally require a lower latency line than Video-On-Demand (VOD), but are relatively more tolerant to high error rates). Therefore, if the whole frequency range is subject to a high level of error protection to ensure that VOD services are of sufficient quality, the induced delay from this error protection may increase the latency on the line to an intolerable level for VoIP services.
The ITU-T G.992.1 and G.992.3 standards (the “ADSL” and “ADSL2” standards, respectively) both addressed this problem through channelization. The ADSL standard included the concept of “dual latency paths” which divided the spectrum into two channels having different configuration settings (e.g. different levels of error protection). The ADSL2 standard developed this concept further to define “multiple latency paths”, in which up to four different channels could be configured with different configuration settings. Once created, the channels were applied to the line and data for a particular service was loaded onto a particular channel depending on its operational requirements.
U.S. Pat. No. 7,813,434 discusses the concept of multiple latency paths in more detail. In this disclosure, the DSL line is divided up into several channels, and the DSL modem determines the transmission characteristics of each channel. The DSL modem may then transmit data on the line by loading data onto channels having corresponding transmission characteristics (e.g. data for a VoIP service will be loaded onto a channel having low latency).
Although this feature was not commonly used in low bandwidth DSL systems, the present inventors have identified that the concept of channelization of DSL technologies can be improved (in particular for high frequency systems such as G.Fast).